A communication system includes DL (DownLink) and UL (UpLink). The downlink is a connection for delivering signals from one or more TPs (Transmission Points) to UEs (User Equipments). The uplink is a connection for delivering signals from UEs to one or more RPs (Reception Points). Normally, UE is also referred to as a terminal or a mobile station. UE may be a fixed type or a mobile type. For example, UE may include a wireless device, a cellular phone, or a personal computing device. In general, TP or RP is a fixed station. TP and RP may be formed as a single integrated apparatus, which may be referred to as a base station. Also, this base station may be referred to as a BTS (Base Transceiver System), a node B, an eNB (enhanced Node B), an AP (Access Point), or the like.
A communication system supports the transmission of several signal types including a data signal, a control signal and a reference signal. A data signal delivers information contents. A control signal allows an appropriate processing of data signals. A reference signal is also referred to as a pilot, and allows a coherent demodulation of data or control signals. Once a reference signal is transmitted, channel state information (CSI) corresponding to an estimated value of a channel medium may be created.
UL data information is delivered through PUSCH (Physical Uplink Shared CHannel). UCI (Uplink Control Information) is delivered through PUCCH (Physical Uplink Control CHannel), except case in which UE has PUSCH transmission, and UE can deliver at least part of UCI as well as data information through the PUSCH. UCI includes ACK (ACKnowledgement) information associated with use of HARQ (Hybrid Automatic Repeat reQuest) process. HARQ-ACK is a response to reception of TBs (Transmission Blocks) by UE at DL of a communication system, and this corresponds to signal transmission from a node B to UE.
DL TBs are transmitted through PDSCH. UCI may include a CQI (Channel Quality Indicator), a PMI (Precoding Matrix Indicator), or an RI (Rank Indicator). CQI, PMI and RI may be collectively referred to as CSI (Channel State Information). CQI offers, to a node B, a measured value of SINR (Signal to Interference and Noise Ratio) experienced by UE through sub-bands or the entire operating DL BW (BandWidth). Typically, this measured value is the form of the maximum MCS (Modulation and Coding Scheme) that can be achieved by a predetermined BLER (BLock Error Rate) about transmission of TBs. Through PMI/RI, a node B may be notified of a method for combining signal transmission from UE to node B antennas according to MIMO (Multiple-Input Multiple-Output) mode. UE can transmit UCI through PUCCH separately from data information. Alternatively, UE may transmit UCI through PUSCH together with data information.
DL data information is delivered through PDSCH. DCI (Downlink Control Information) includes a DL CSI feedback request to UEs, UL SAs (Scheduling Assignments) about PUSCH transmission to UEs, or DL SAs about PDSCH reception by UEs. SAs are delivered through DCI formats transmitted through respective PDCCHs (Physical Downlink Control CHannels). In addition to SAs, PDCCHs may deliver DCI common to all UEs or one group of UEs.
Additionally, DCI includes HARQ-ACK information transmitted to UEs by one or more TPs through PHICHs (Physical HARQ-ACK Indicator CHannels) in response to reception of data TBs transmitted from UEs to RPs.
Typically, PDCCHs are major parts of a total DL overhead. One method for reducing this overhead is to scale down its size according to resources required for transmission of PDCCHs and PHICHs. If OFDMA (Orthogonal Frequency Division Multiple Access) is used as a DL transmission method, a CCFI (Control Channel Format Indicator) parameter may be transmitted through PCFICH (Physical Control Format Indicator CHannel) and represent the number of OFDM symbols allocated to a DL control region during DL TTI (Transmission Time Interval).
FIG. 1 shows a structure of a control region in DL TTI.
In FIG. 1, a single subframe is composed of M symbols. Referring to FIG. 1, a DL control region occupies the first N subframe symbols 110. The rest M-N subframe symbols 120 are assumed to be used mainly for PDSCH transmission. PCFICH 130 is transmitted at some sub-carriers of the first symbol. Sub-carriers are also referred to as resource elements (REs). It is supposed that PCFICH delivers two bits that represent a PDCCH size of subframe symbols (M=1, 2, or 3). Additionally, PHICH 140 is transmitted at some REs of the first subframe symbol. Further, some subframe symbols include RS REs 150 and 160 common to all UEs with regard to each of transmission antennas assumed to be two in FIG. 1. The central aim of UE-CRS (Common RS) is to allow UE to acquire a channel estimation value about a DL channel medium and also to perform other measurements and functions well known in the art. The rest REs in the DL control region are used for transmission of PDCCH.
PDCCH which delivers SAs is not transmitted at a predetermined location of the DL control region. As a result, each UE should perform a plurality of decoding operations so as to determine whether there is SA in a DL subframe. In order to assist such decoding operations of UE, REs that deliver respective PDCCHs are grouped into CCEs (Control Channel Elements) in the logical domain. Regarding a given number of DCI format bits, the number of CCEs for DCI format transmission depends on a channel coding rate (assuming QPSK (Quadrature Phase Shift Keying) is a modulation type). For UEs that experience lower or higher SINR in DL, serving TPs may use a lower or higher channel coding rate about PDCCH transmission so as to achieve a desired BLER. Therefore, PDCCH transmission to UE experiencing a lower DL SINR may often require much more CCEs than RDCCH transmission to UE experiencing a higher DL SINR does (different power boosting may be also used for REs of CCE transmission). Normal CCE aggregation levels for PDCCH transmission are, for example, one, two, four, and eight CCEs.
Regarding PDCCH decoding process, UE may determine a search space for candidate PDCCHs after CCEs are restored in the logical domain, depending on a common set of CCEs for all UEs (UE-CSS (Common Search Space)) or a UE dedicated set of CCEs (UE-DSS (Dedicated Search Space)). UE-CSS may be formed of the first NCCEUE-CSS CCEs in the logical domain. UE-DSS may be determined according to a pseudo random function that has, as inputs, a UE common parameter such as the number of subframes or the number of total CCEs in a subframe, and a UE specific parameter such as UE identity (UE_ID).
For example, with regard to CCE aggregation level LE ∈{1,2,4,8}, CCEs for PDCCH candidate m are given as L×{(Yk+m) mod(floor(NCCE,k/L)}}+i. Here, NCCE,k is the total number of CCEs in a subframe k. Also, i ranges from 0 to L−1. Also, m ranges from 0 to M(L)−1. Also, M(L) is the number of PDCCH candidates for monitoring in a search space. Also, floor(x) is a function of returning the maximum integer smaller than or equal to x. Meanwhile, floor(x) may be expressed as └x┘. Two expressions will be used together hereinafter. Exemplary values of M(L)(L∈{1,2,4,8}) are {0, 0, 4, 2} in UE-CSS and {6, 6, 2, 2} in UE-DSS. For UE-CSS, Yk=0. For UE-DSS, Yk=(A×Yk-1)mod(D), where Y−1=UE_ID≠0, A=39827, and D=65537.
For example, PDCCHs which deliver information to a plurality of UEs, such as PDCCH which delivers TPC (Transmission Power Control) commands for UEs so as to regulate PUSCH or PUCCH transmission power, are transmitted at UE-CSS. Additionally, if there are sufficient CCEs in UE-CSS after transmission of PDCCHs which deliver DCI to UEs at a subframe, UE-CSS may be also used for transmitting PDCCH which delivers some SAs having specific DCI formats. UE-DSS is used exclusively only for transmitting PDCCHs which offer SAs. For example, UE-CSS may be formed of 16 CCEs, and hence support two PDCCHs having CCEs of L=8, support four PDCCHs having CCEs of L=4, support one two PDCCHs having CCEs of L=8, or support two PDCCHs having CCEs of L=4. CCEs for UE-CSS are disposed first in the logical domain (before interleaving).
FIG. 2 shows a PDCCH transmission process.
Referring to FIG. 2, after channel coding and rate matching, encoded bits of DCI formats are mapped to CCEs in the logical domain. The first four CCEs (L=4), i.e., CCE1 201, CCE2 202, CCE3 203, and CCE4 204, are used for PDCCH transmission to UE1. The next two CCEs (L=2), i.e., CCE5 211 and CCE6 212, are used for PDCCH transmission to UE2. The next two CCEs (L=2), i.e., CCE7 221 and CCE8 222, are used for PDCCH transmission to UE3. Finally, the last CCE (L=1), i.e., CCE9 231, is used for PDCCH transmission to UE4. At step 240, DCI format bits of PDCCH may be scrambled with a binary scrambling code. Scrambled DCI format bits are modulated at step 250. Each CCE is further divided into REGs (Resource Element Groups). For example, CCE formed of 36 REs may be divided into 9 REGs, each of which is formed of 4 REs. At step 260, interleaving is applied to REGs (blocks of four QPSK symbols). For example, block interleaving that interleaving is performed for symbol-quadruplets (4 QPSK symbols corresponding to 4 REs of REG) instead of individual bits may be used. After REG interleaving, consequence series of QPSK symbols may be shifted by J symbols at step 270. Finally, at step 280, each QPSK symbol is mapped to RE in a DL control region of subframe. This mapping is made first in a frequency direction and then made in a time direction. Therefore, in addition to RS 291 and 292 from transmission antennas and other control channels such as PCFICH or PHICH 293, REs in a DL control includes QPSK symbols corresponding to DCI format for UE1 294, UE2 295, UE3 296 and UE4 297.
FIG. 3 shows exemplarily a PUSCH transmission structure. For simplicity, TTI is formed of a single subframe 310 including two slots. Each slot 320 contains NsymbUL symbols used for transmission of data signals, UCI signals, or reference signals (RS). Each symbol 330 relieves interference due to channel propagation effects, including CP (Cyclic Prefix). PUSCH transmission in one slot may use the same BW as or different BW from PUSCH transmission in the other slot. Some symbols in each slot are used for transmission of RS 340 that allows coherent demodulation and channel estimation of received data and/or UCI signals. Transmission BW is formed of frequency resource units which are referred to as PRBs (Physical Resource Blocks). Each PRB is formed of NSCRB sub-carriers or resource elements (REs), and UE is allocated MPUSCH PRBs 350 for total MSCPUSCH=MPUSCH×NSCRB with regard to PUSCH transmission BW. The last subframe symbol may be used for transmission of SRS (Sounding RS) 360 from one or more UEs. The central aim of SRS is to offer a CQI estimation value to a node B for UL channel medium regarding each UE. SRS transmission parameters for each UE are formed semi-statically by a node B through upper layer signaling. The number of subframe symbols capable of being used for data transmission is NsymbPUSCH=2(NsymbUL−1)−NSRS. Here, NSRS=1 if the last subframe symbol is used for SRS transmission, and Nms=0 in the other case.
FIG. 4 is a block diagram of a PUSCH transmitter. A multiplexer 420 multiplexes coded CSI bits 405 and coded data bits 410. Then, an HARQ-ACK insertion unit 430 punctures data bits and/or CSI bits and inserts HARQ-ACK bits. Then, a DFT unit 440 performs DFT (Discrete Fourier Transform) of data into which HARQ-ACK bits are inserted. A mapping unit 450 selects (mapping) REs corresponding to PUSCH transmission BW, and a transmission BW control unit 455 controls transmission BW. An IFFT unit 460 performs IFFT (Inverse Fast Fourier Transform) of mapping signals. Finally, a CP insertion unit 470 inserts CP into IFFTed signals. A time windowing unit 480 performs filtering. A filtered transmission signal 490 is delivered to a receiver. For simplicity, additional transmitter circuitry such as a digital-analog converter, analog filters, amplifiers, and transmitter antenna is not illustrated. Also, for simplicity, an encoding process for data bits and CSI bits as well as a modulation process for all transmission bits are omitted. It is supposed that PUSCH transmission is made on clusters of adjacent REs or a plurality of non-adjacent clusters 495B according to DFT-S-OFDM (DFT Spread Orthogonal Frequency Multiple Access) method (or also referred to as SC-FDMA (Single-Carrier Frequency Division Multiple Access)) allowing signal transmission through one cluster 495A.
FIG. 5 is a block diagram of a receiver. In the receiver, reverse (complementary) operations of transmitter operations are performed. Reverse operations of operations shown in FIG. 4 are shown in FIG. 5. After an antenna receives a radio frequency (RF) analog signal, and after processing of additional processing units (such as filters, amplifiers, frequency lowering converters, and analog-digital converters) not shown for simplicity, a time windowing unit 520 filters a received signal 510. A CP elimination unit 530 removes CP from the filtered signal. Then, an FFT unit 540 applies FFT to the CP-removed signal. A demapping unit 550 selects (demapping) REs 560 used by the transmitter. An IDFT unit 570 applies IDFT to the demapped signal. An ACK/NAK extraction unit 570 extracts ACK/NAK. A demultiplexing unit 580 demultiplexes data bits 590 and CSI bits 595. Like the transmitter, well-known receiver functions such as channel estimation, demodulation, and decoding are not illustrated for simplicity.
In order to support a higher data rate than what is available in legacy communication systems, a plurality of CCs (Component Carriers) (also referred to as CA (Carrier Aggregation)) are considered in both DL and UL and thereby offer higher operating BWs. For example, in order to support a communication through 60 MHz, an aggregation of three 20 MHz CCs may be used.
FIG. 6 shows the principle of CC aggregation. An operating DL BW 610 of 60 MHz is formed of an aggregation of three DL CCs 621, 622 and 623 (shown as continuity for simplicity) each of which has 20 MHz BW. Similarly, an operating UL BW 630 of 60 MHz is formed of an aggregation of three UL CCs 641, 642 and 643 each having 20 MHz BW. For simplicity, it is supposed in FIG. 6 that each DL CC is mapped inherently to UL CC (symmetric CC aggregation). However, it is possible to map two or more DL CCs to a single UL CC or to map two or more UL CCs to a single DL CC (non-symmetric CC aggregation not shown for simplicity). Typically, links between DL CCs and UL CCs are UE-specified type.
A node B forms CCs for UE, using upper layer signaling such as, e.g., RRC (Radio Resource Control) signaling. DL CCs formed by RRC may be activated or inactivated by MAC (Medium Access Control) signaling or physical layer signaling (Activation/inactivation of UL CC formed by each RRC is determined depending on activation/inactivation of linked DL CC). The activation of DL (UL) CC for UE means that UE can receive PDSCH (transmit PUSCH) at the CC, and the reverse is applied to the inactivation of DL (UL) CC. To maintain a communication, one DL CC and one UL CC linked thereto are needed to remain in an activation state, and these will be referred to as DL PCC (DL Primary CC) and UL PCC (UL Primary CC), respectively.
An aperiodic CSI report through PUSCH is triggered by a CSI request field in PDCCH. In the following description, a serving cell corresponds to each component carrier (CC). When a transmitted display is decoded in scheduling allowed for a serving cell (c), an aperiodic CSI report is performed using PUSCH on the serving cell (c). If the size of a CSI request field is one bit, a report is triggered in case the CSI request field is set to ‘1’. If the size of a CSI request field is two bits, a report is triggered according to Table 1 given below.
TABLE 1Value of CSIRequest FieldDescription‘00’No aperiodic CSI report is triggered‘01’Aperiodic CSI report is triggered for a serving cell (c)‘10’Aperiodic CSI report is triggered for the first set ofserving cells formed by upper layers‘11’Aperiodic CSI report is triggered for the second set ofserving cells formed by upper layers
For example, in case a carrier indicator field (CIF) is 1 (bit ‘001’) and a CSI request field is bit ‘01’, CSI of DL CC1 linked to UL CC1 feeds back to a node B due to CIF. In case a CSI request field is bit ‘10’, CSI(s) of DL CC(s) feeds back to a node B, depending on upper layer configuration.
FIG. 7 shows a resource structure of LTE-A. DL transmission of LTE and LTE-A are implemented by the subframe in the time domain and by the RB in the frequency domain. While subframe is equal to 1 msec of a transmission period, RB is equal to 180 kHz of a transmission bandwidth formed of 12 sub-carriers. As shown in FIG. 7, a system bandwidth of LTE-A is formed of a plurality of RBs in the frequency domain and formed of a plurality of subframes in the time domain.
Many different signals are transmitted for LTE-A release 10 and the next releases. In DL, the following reference signals are transmitted:
1. Cell-specific reference signal (CRS): Used for initial system access, paging, PDSCH demodulation, channel measurement, handover, or the like.
2. Demodulation reference signal (DMRS): Used for demodulation of PDSCH
3. Channel state information reference signal (CSI-RS): Used for channel measurement.
In addition to the above reference signals, zero power CSI-RS may be applied to LTE-A release 10. Zero power CSI-RS may occur at the same time and frequency resources as CSI-RS does, but may be different from CSI-RS in that there is no signal transmitted onto REs dependent on zero power CSI-RS. The aim of zero power CSI-RS is not to create interference on CSI-RS transmitted by adjacent TPs through no transmission onto resources used by adjacent TPs for CSI-RS transmission of specific TP.
FIG. 8 is a configuration diagram of resources in LTE or LTE-A system. Referring to FIG. 8, the locations of resources used for transmission of different reference signals, PDSCH, zero power CSI-RS, and control channels are shown. It should be noted that FIG. 8 relates to a single RB in the frequency domain and to a single subframe in the time domain. A plurality of RBs may exist for each subframe, and the above signals may be transmitted on a plurality of RBs in a similar manner as shown in FIG. 8. Resources marked with alphabets A, B, C, D, E, F, G, H, I and J in FIG. 8 correspond to the locations where transmission for CSI-RS has four antenna ports. For example, in four REs marked with ‘A’, CSI-RS having four antenna ports may be transmitted. CSI-RS having two antenna ports may be transmitted on resources acquired by restricting resources for CSI-RS having four antenna ports to two. Additionally, CSI-RS having eight antenna ports may be transmitted on resources acquired by combining two resources for CSI-RS having four antenna ports. Zero power CSI-RS may be applied to resources for CSI-RS having four antenna ports.
In DL transmission mode 9 of 3GPP LTE-A release 10, UEs measure CSI-RS transmitted by eNB, and also create and feed back a DL CSI such as RI (Rank Indicator), PMI (Precoding Matrix Indicator), and CQI (Channel Quality Indicator). Each of RI, PMI and CQI is reported at individual timing displayed by eNB. In a CSI feedback, PMI is calculated on the basis of the most recently reported RI, and CQI is calculated by assuming the most recently reported RI and PMI.
The central aim of communication systems is to improve a coverage and cell-edge throughput. CoMP (Coordinated Multi-Point) transmission/reception is an important technique for achieving this aim. In case UE is in a cell-edge region, CoMP operation depends on the fact that it can reliably receive signals from a set of TPs (DL CoMP) and also reliably transmit signals to a set of RPs (UL CoMP). DL CoMP methods may include more complicated methods that require exact and detailed channel information, such as joint transmission from a plurality of TPs, as well as a simple method of interference avoidance such as adjusted scheduling. Additionally, UL CoMP methods may include more complicated methods of considering received signal characteristics and created interference at a plurality of RPs as well as a simple method of performing PUSCH scheduling in consideration for a single RP.